This invention relates to thermophotovoltaic power generators for converting fuel to electricity using no moving parts.
We have previously demonstrated that the use of low band gap photovoltaic cells such as GaSb cells instead of conventional silicon solar cells allows the use of lower temperature infrared emitters. Specifically, infrared emitters operating at temperatures in the range of 1700K to 2000K are well suited for use with GaSb cells. However, in our previously issued patents and our recently filed patent applications, we have described TPV generators designed for high efficiency conversion of fuel energy into electrical energy. This leads to burner configurations designed specifically for TPV incorporating heat exchangers, infrared filters, and special ceramic emitters. Although applications exist for these high efficiency TPV generators, it is also desirable to add TPV generators onto existing burner configurations where the burner is already designed and available for another primary purpose. Examples of such simple burners include candles or oil lamps designed primarily for lighting as well as gas burners designed for space heating or water heating. In mountain cabins, for example, propane wall heaters will be used in the winter for space heating. Why not generate electricity as well?
There is a need to incorporate low band gap cells into existing burner configurations in order to add the capability of electric power production to a burner designed primarily for another purpose. This adds utility to the existing burner while allowing the TPV circuit manufacturer to focus on his area of expertise while obtaining the use of a burner essentially for free.
We began the work which led to this patent application by observing that the yellow flame from a candle or oil lamp when surrounded by a small bracelet containing GaSb cells provided enough energy to operate a small transistor radio. Our analysis showed that the energy emitted by the flame was primarily in the infrared, peaking at 1.3 microns, ideal for the GaSb cells which responded out to 1.7 microns. Our analysis further showed that the infrared emitted originated from small carbon particles burning in the flame at a temperature of 2200K. However, although the flame temperature was definitely hot enough, the carbon particle density was very low, resulting in a relatively small amount of total emitted infrared energy. The result was a nice demonstration unit producing too little power to be economically viable. The power produced was only 0.1 Watts.
There is a need to dramatically increase emitter surface area in order to increase IR intensities, thereby increasing generated electrical power without dramatically increasing burner complexity or cost.
Attempts have been made to increase the amount of emitted TPV cell convertible infrared by inserting ceramic fiber mantles containing rare earth oxides into hydrocarbon burner flames. The idea is based on the fact that hot erbium oxide, for example, emits infrared at a wavelength of 1.55 microns which can be converted to electricity by a GaSb TPV cell. It is well known, however, that these ceramic mantles are very fragile. In addition, we have observed that the infrared emitted from an erbia mantle in a hydrocarbon flame contains large infrared energy peaks at 2.7 and 4.5 microns which are associated with gas phase CO.sub.2, CO, and H.sub.2 O molecular vibrations. It is therefore important to distinguish between combustion occurring on the emitter surface with the resultant emission of convertible IR and combustion occurring in the gas phase with the emission of non-useful IR.
There is a need to identify an IR emitter which is not fragile. Furthermore, this emitter should promote surface combustion reactions allowing for good energy transfer to its surface. Finally, it should emit primarily TPV cell convertible IR.
For improved economics and for expanded utility, needs have arisen for thermophotovoltaic generators having outputs in the range of one Watt or more and adequate heat removal. Efficient emitters must be capable of withstanding high temperatures and must efficiently couple energy from combustion into IR radiation usable by photovoltaic cells.
Needs exist for small, inexpensive thermophotovoltaic electric generators using conventional burners equipped with efficient IR emitters having a high IR power output well matched to existing low band gap cells.
There also is a need for electric power generation along gas pipe lines requiring small size, simple thermophotovoltaic units.